The present invention relates to an imaging method and apparatus for use in an apparatus for radiographing a section of a human body using a nuclear magnetic resonance (hereinafter, abbreviated as an NMR) phenomenon and is used for medical diagnosis.
After the NMR imaging was proposed by Lauterber in 1973, the imaging methods for extension and modification of the NMR imaging have been published. The proton spin density and the values of relaxation times T.sub.1 and T.sub.2, or the images of which the spin density was changed by the relaxation time are obtained as image information. Not only the morphological information but also the functional or biochemical information are derived due to those images; therefore, the NMR imaging is strongly expected for the malignant tumor diagnosis and energy metabolism diagnosis.
In addition, the blood flow information can be also obtained from the NMR images (L. E. Crooks et al: "Visualization of Cerebral and Vascular Abnormalities by NMR Imaging. The Effects of Imaging Parameters on Contrast", Radiology 144: 843-852). According to the NMR imaging, only the hydrogen atomic nuclei in the special slice of the human body are excited and the electromagnetic waves which are irradiated therefrom are observed. However, in the blood flow portion, the excited atoms flow out of the slice plane, so that the signal intensity decreases. The above-mentioned literature intends to know the blood flow velocity using the foregoing nature.
This method has the following drawbacks.
(1) The doctor is required to specify the location of the blood flow portion (blood vessel). PA1 (2) There isn't the one-to-one correspondence relation between the blood flow velocity and the signal intensity. PA1 (3) The direction of blood flow is unclear.
First, in the problem of (1), although the blood flow portion also has a specific density this density varies depending on the flow velocity. Density represents a degree of optical light and dark parts of an original image visually perceived by a human. When the original image is displayed on a screen, the image is converted into a signal intensity corresponding to the depth of monochromatic or color image in which the signal is modulated in accordance with the intensity thereof. Further, the concentration range overlaps the concentration ranges of the other portions. Therefore, the location of the blood flow portion cannot be specified from only the concentration. Therefore, the doctor needs to instruct the objective blood flow portion by observing the images.
This means that the blood flow velocity cannot be automatically measured and, further, there is a possibility such that the range and portion which are instructed by the doctor or patient change and reproducibility lacks, causing inconvenience for the time-sequential observation.
The problem of (2) is important since it further influences on the precision than the problem of (1). Although the density of the blood flow portion decrease as mentioned above, when speaking accurately, the density decreases after it has once increased (refer to the foregoing literature). This phenomenon, as shown by experimental data in FIG. 1, occurs due to the following reasons.
In the NMR imaging, the hydrogen atomic nuclei are excited and the electromagnetic waves which are irradiated therefrom are observed. This operation is ordinarily performed hundreds of times. Although it is desirable to observe after the atomic nuclei which had once been excited were returned to the normal state, this results in an increase in observation time; therefore, the observation is continued at proper time intervals. Consequently, the observation is carried out before the atomic nuclei are returned to the normal state and this causes the signal intensity to deteriorate.
However, in the blood flow portion, the excited atomic nuclei flow out of the slice plane and at the same time, the fresh atomic nuclei which were not excited in the previous observation flow into this blood flow portion. In the range of small blood flow velocity, the density once increases because of the increase in density due to the inflow of the fresh atomic nuclei rather than the reduction in density due to the outflow of the excited atomic nuclei. When the blood flow velocity exceeds a predetermined value, the influence of the outflow of the atomic nuclei becomes large and the density monotonously decreases.
In other words, the same density is presented at different blood flow velocity and there isn't the one-to-one correspondence relation between the velocity and the density (signal intensity). On the contrary, in the low blood flow velocity range, the density slightly changes and the sensitivity is also bad.
Lastly, the problem of (3) is caused when attention is paid to only the reduction of the signal intensity.
The above-mentioned problems of the conventional technology practically become various obstacles to development of the NMR imaging.